There has been recently an increasing demand for an NC lathe provided with a turret device since a machining apparatus, particularly, a lathe is able to realize automatic machining operations at high speed and can cope with a computer-integrated flexible manufacturing system.
The turret device is structured such that a rotary table called a turret which is provided on a tab base table (tool slide), and has normally 5 to 12 indexing positions, is indexed while rotated (hereinafter referred to as simply indexed). Tool mounting portions are formed on an outer periphery or end surface of the turret, and various tools such as turning tools and drills are mounted on these tool mounting portions wherein an appropriate tool is selected from the various tools depending on the type of machining process desired so as to index the selected tools in machining positions.
In indexing devices such as the turret, a pair of positioning engaging members which are engageable with and disengageable from each other are provided so that the selected tool is indexing in a given indexing position, and the tool which was arranged in the indexing position is positioned and fixed to the indexing position.
This pair of positioning engaging members is provided between the base table and a turret, wherein the turret may be freely indexing or may be positioned and fixed to a given indexing position when the positioning engaging members engage with or disengage from each other.
In the turret device of the NC lathe, crown gears having inclined tooth profiles such as curvic couplings which mesh with each other are generally employed as the positioning engaging members. In the positioning engaging member of this type, when a turret side coupling is moved in the axial direction relative to a base table side coupling, the engagement and disengagement between these couplings is performed.
The positioning engaging members of this type are needed to apply a large pressing force in the axial directions thereof to maintain the engagement therebetween owing to the meshing between the tooth profiles. If the pressing force in the axial direction is small, there occurs a situation that one of the positioning engaging members rises along each tooth flank of the inclined tooth profile under the load applied to the turret, for example, by the cutting resistance which is applied to the tools mounted on the turret.
Accordingly, a sufficiently large pressing force has been conventionally applied to the positioning engaging members in the axial direction thereof, thereby preventing the positioning engaging members from rising under the load.
However, energy is wasted when the sufficiently large pressing force is applied to the positioning engaging members in the axial direction thereof even if no load is applied to the turret (for example, when the tool mounted on the turret does not perform the cutting operation) or when the load is small. Further, if such a large pressing force is applied, a large surface pressure is generated between tooth flanks of the positioning engaging members, which causes abrasion and increases the temperature of the driving source of the pressing force, thereby generating heat and causing heat deformation.
The present invention has been made in view of these circumstances, and it is the object of the invention to control the pressing force applied to the positioning engaging members appropriately in response to the load applied to the rotary table.